Dry grinding feed control



Oct. 16, 1956 D. WESTON DRY GRINDING FEED CONTROL 2 Shets-Sheet 1 Filed April 30, 1954 a w w R A c E J 6 w N o n P w m w m V: a I R r w T T M N O m C V D E m w m 0 n L N S 0 w v 5 E G m L m m T M S C w w W i 359 $201 2252328 $28 A? 5 550m .3 a wasp ZOGwJZw QZDOW QDQI Emu WZOF I'll-I'l 14 13 COMPARATOR AMPLIFIER PROPORTON FEEDER CONTROL SIGNAL SIGNAL REFERENCE 5.1 PROPORTION FEEDER 15 AMP/LIFIER MONITOR/7,16

COMP/1 RATOR CONTROL SIGNAL REFERENCE SIGNAL v Oct. 16, 1956 D. WESTON DRY GRINDING FEED CONTROL 2 Sheets-Sheet 2 Filed April 30, 1954 INVE/VTGR DA VID WES TON B w ggww ATTORNEYS.

United States Patent 2,7 66,941 DRY GRHNDING FEED CGNTROL David Weston, Toronto, Ontario, Canada Application April 30, 1954, Serial No. 426,721

9 Claims. c1. 241 s0 This invention relates to the automatic control of material reduction mills of the type adapted to reduce material in its dry state.

Many types of dry grinding mills are known such as dry ball mills, pebble mills, tube mills, and the like, as well as combined dry crushing and grinding mills of the general type described in my prior Patent No. 2,555,171.

All of the above-mentioned mills have in common the fact that over their normal range of operating conditions the amount of power consumed by the mill motor will vary in characteristic fashion depending upon the amount of material in the mill, while at the same time the intensity of the sound produced Will also vary in characteristic fashion in accordance with the load within the mill.

The above observations have been common knowledge in the art for some considerable time and various attempts have been made to control the feed to the mill automatically on the basis of sound emitted by the mill or on the basis of power consumed by the mill motor. To date, the systems of control proposed in the art have lacked any appreciable advantage over manual control by a skilled operator by reason of the fact that at the point of maximum capacity for any given type of dry mill, the characteristic curve of power consumption versus charge volume exhibits a peak and since it is usually desired to maintain maximum capacity, it was not possible to base the automatic control of the feed upon a control signal derived from the power consumption of the mill motor because the required direction of application of the correction to the feed rate was not ascertainable. In other words, on the basis of motor horsepower, there was no way of knowing whether the charge volume in the mill was too great or too small.

In the case of sound, previous attempts at controlling the feed to mills on the basis of the sound emitted by the mill have not produced the advantages forecast for them because of a lack of appreciation of the various factors involved and a failure to apply the control signal derived from the sound in a manner which provided for a steady operation of the mill at the operational point desired. Previous types of control have given rise to serious conditions of overload and underload at the desired operating point, and it has not been realized previously that a considerable loss of eificiency arises by reason of such conditions even if the main point of operation coincides substantially with optimum operating conditions.

In my copending applications Serial Nos. 259,060 (filed November 30, 1951), now abandoned, and 282,505 (filed April 15, 1952), I describe a method of elfecting control of material reduction mills on the basis of a combination of a signal which is proportional to the power input to the mill with a signal which is inversely proportional to the rate of production of the mill. According to the teachings of my said prior applications, I am able to hold a mill constantly at its point of maximum capacity and thereby effect a considerable increase in efficiency.

In many instances, however, it is desired to operate mills at points other than the point of maximum capacity ice for reasons which will be hereinafter explained. I have discovered that satisfactory control of dry material reduction mills at any desired operating point can be effected by the use of control signals derived either from the power consumed by the mill motor or from the sound emanating from the mill, depending upon the particular operating point desired, provided the control signal is applied in a manner which varies the rate of feed proportionally in accordance with the extent of departure of the actual operational point of the mill from the point of operation which it is desired to maintain.

In order to select the signal best adapted for use as a control signal in any particular instance, it is necessary to consider three characteristic curves of the mill in question when operating upon the particular material which it is desired to comminute. These three characteristic curves and the functioning of the invention will be explained in conjunction with the drawings Wherein Figure 1 is a graph illustrating the three characteristic curves referred to;

Figure 2 is a block diagram illustrating the control system of the invention;

Figure 3 is a block diagram illustrating a control system according to the invention where the system is monitored;

Figure 4 diagrammatically illustrates the components of the sound signal source;

Figure 5 is a circuit diagram illustrating a suitable means for deriving the power signal;

Figure 6 illustrates a suitable circuit arrangement for the application of a monitoring signal to the system.

Referring to Figure 1, curve A represents what is termed in the art the grinding curve. This curve represents tons per hour versus charge volume, and, although its shape will vary from mill to mill its characteristics are generally the same for all mills, namely tons per hour commence at zero at the point of origin, increase to a maximum as the total charge volume increases and then decrease. In grinding mills such as ball mills, tube mills, pebble mills and the like where the degree of comminution is fixed by the characteristics of the grinding media used and the particle size distribution of the feed, the product produced by the mill is substantially the same regardless of the point on the grinding curve at which the mill is operated, and, therefore, it is the usual practice to operate the mill insofar as it is possible at a point which corresponds to the peak of the grinding curve.

In combined dry crushing and grinding mills, however, and in particular in mills of the type described in my prior Patent No. 2,555,171, the size distribution of the product produced is difierent for each point on the grinding curve, and, therefore, each point on the grinding curve for such mills also represents a particular degree of cornminution of the feed material.

The second characteristic curve is that of power consumption versus charge volume, and is represented by curve B and referred to herein as the power curve.

The power curve is generally similar in shape to the grinding curve and has the same characteristic peak in it although, since what is being plotted against charge volume is power consumption, the peak of the power curve does not necessarily occur at the same charge volume as does the peak on the grinding curve. Generally speaking, however, in ball mills, the peak on the power curve will occur at a slightly greater charge volume than the peak on the grinding curve. Unlike the grinding curve, however, the power curve does not pass through the origin inasmuch as a certain amount of power is required to turn the mill over when it is empty.

The third characteristic curve is that of sound of vibration produced by the mill during operation versus charge volume. In Figure 1, this is represented by curve C, and

3 v it is plotted as an inverse function for reasons which will be explained later. Sufiicient to say at this point that the curve represents the inverse function of the voltage produced by sound or vibration plotted against charge volume on the same base as curves A and B. It will be observed that inverse sound volts commence at a low value and gradually increase as the charge volume increases and that the steepness of the curve rapidly increases until it is nearly vertical for large charge volumes. In other words, the mill is noisiest for low charge volumes and becomes quieter with increases in charge volume until a point is reached where the mill is comparatively silent when the mill is overloaded.

Curve C is generally of the same characteristic shape for all types of mill and may represent sound produced by the mill, vibration produced by the mill, or may represent sonic or non-sonic vibration produced by the mill within selected frequency ranges.

It will readily be observed that the steepest part of the power curve occurs in the region of low charge volumes, whereas the steepest part of the sound curve occurs in the region of high charge volumes. When either of these curves is steep, it means that a relatively small change in charge volume corresponds to a relatively large change in power consumption or sound emission as the case may be. Accordingly, when a curve is steep, it is most ideally suited as a control signal, and accordingly, in accordance with the present invention, if it is desired to operate a mill with a large charge volume, or near its point of maximum capacity, I use sound or vibration as the source of the control signal. On the other hand, if the mill is to be operated at a small charge volume, I prefer to use power consumption as the source of the control signal. It will be appreciated that sound could be used at lower charge volumes because the sound curve always does have some slope. It is, however, not nearly as steep at low charge volumes as is the power curve, and, therefore, if the control signal is derived from sound when operating at low charge volumes, the control effected will not be as precise and hunting will to some extent be aggravated. In carrying out the method of the present invention, I first of all select the point on the grinding curve at which it is desired to operate. I then compare the steepness of the power curve at the same charge volume with the steepness of the sound curve and select the signal represented by the steeper curve as the source of the control signal. I then determine either empirically or from available data the value of the control signal at the desired point of operation and use this as the value of a reference signal which is continually produced from an independent source. I then continually produce the control signal and compare it to the reference signal to produce a ditference signal, and I vary the rate of feed to the mill in accordance with the sense and magnitude of the difference signal, thus keeping the operation of the mill comparatively steady at a point which corresponds to the point which I originally selected on the grinding curve.

Referring to Figure 1 and assuming that I desire to operate a mill at a point corresponding to point X on the grinding curve, by drawing a line vertically through the point, it is a simple matter to determine by inspection that the'slope of the sound curve at point Y is greater than V the slope of the power curve at point Z. I would, therefore, select sound or vibration as the source of the control signal and use as a reference signal a voltage corresponding to the distance YW.

Similarly, if it is desired to operate the mill at a point corresponding to point P on the grinding curve, it is once again a simple matter to determine by visual inspection that the slope of the power curve at point Q is greater than the slope of the sound curve at point R, and, therefore, I would select power as the source of the control signal and use as a reference signal a voltage which corresponds to a voltage which corresponds to the distance QS. From what has been 'said above, it will readily be appreciated that if I desire to operate the mill at a point a reference is exceeded.

which corresponds to point T on the grinding curve, I could use either sound or power as the source of the control signal equally as effectively since the slope of the power curve at point U is substantially the same as the slope of the sound curve at point V.

In some cases, it may be desirable to monitor the control system to avoid overloading the mill motor or to avoid exceeding any other predetermined fixed conditions of the milling circuit which it is considered important should not be exceeded. In such cases, a monitoring signal which is functional to or varies with the condition of operation on the basis of which it is desired to monitor is applied to the system in such a manner that whenever the said condition is exceeded the rate of feed to the mill is diminished regardless of the sense and magnitude of the difference signal referred to above. 7

Where the physical characteristics of the ore fed to the mill are subject to variation so that the power consumption at a given charge volume is subject to variation, it may be of advantage when using power as the source of the control signal to monitor on the basis of sound to prevent the charge volume from being built up or reduced due to the introduction into the mill of higher or lower specific gravity material.

If the type of mill which it is desired to control is a combined dry crushing and grinding unit of the general type described in my prior Patent No. 2,555,171, the procedure to be followed is substantially the same except that the point selected on the grinding curve representing the desired point of operation of the mill will be selected having regard to the degree of comminution of the product desired rather than upon the basis of therate of production desired. It will be normal in these circum stances to operate the mill at various points on the grinding curve for purposes of determining the point at which the mill produces a product having the desired characteristics. If the mill has previously been calibrated in respect of the particular ore which is being comminuted, the selection may be made on the basis of the available data, and it may not be necessary to determine the desired operational point experimentally. Once the operating point has been selected, the control signal is selected and control is applied in exactly the same manner as previously explained in connection with ball mills and other more conventional forms of mill. 1 I

The control system of the invention is illustrated diagrammatically in Figure 2 where box 10 illustrates a source of a control signal (either power, sound or vibration) and box 11 represents the source of a reference signal. The control signal and reference signal are cornpared in a comparator represented by box 12, and the difference signal thus produced is amplified in amplifier 13. The amplified signal from amplifier 13 is applied to vary the rate at which the feeder 14 feeds the mill 15.

The controlsystern illustrated in Figure 3 is exactly the same as that illustrated in Figure 2 except for the presence of a source of a monitoring signal represented by'box 16 which takes over control of the system and reduces the feed to the mill whenever the monitoring One common form of monitoring is the monitoring of the system to prevent an overload being placed on the mill motor when a control signal is used which is derived from sound produced in the mill. In such a case, the

' monitoring signal simply causes reduction in the rate of feed to the mill whenever the horsepower drawn by the mill motor exceeds a predetermined value (usually its rated horsepower).

Control signal tional to the sound emanating from the mill, the circuit 7 a'iaoii 3 components repfesented by the box 10 in Figures 2 and 3 will consist essentially of a dynamic microphone, an amplifier and a rectifier (as indicated in Figure 4). If it is desired that the signal thus produced be proportional to the sound emitted within only a limited frequency band, the circuit can include a band pass filter, or the elements of the sound pickup system can be selected so that they are very sensitive to the sound frequencies which it is desired to utilize and relatively insensitive to sound frequencies outside the selected range. For instance, it has been found that audible frequencies emitted from a primary ball mill which are above 2,000 cycles per second vary in intensity in close relationship to the actual conditions within the mill Whereas frequencies substantially below 2,000 cycles per second are not very satisfactory as a source of a control signal for purposes of the present invention because an appreciable proportion of the intensity of sound within these low frequencies can be attributed to extraneous causes such as the mechanical noise of the mill and power transfer system.

If the control signal is to be derived from vibrations other than sonic emitted by the mill during operation, apart from the type of pickup used, the circuit components required will be essentially the same as they will be when sound is used. In this case, however, the selection of a predetermined frequency band will more conveniently take place essentially within the circuit rather than as a result of selection of pickup components of selected characteristics.

Where electromagnetic pulse type feeders are employed and the control of the feed rate is elfected by a phase shift in a saturable core reactor controlling thyratron power output tubes associated with the feeder, it will be convenient to invert the voltage of the control signal when it is derived from sound or vibration since the amount of phase shift efiected, and hence the amount of power which is passed to the feeders, is proportional to the voltage applied to the saturable core reactor. (This is apparent from the fact that the sound or vibration produced by the mill will in general be less for high rates of feed than for low rates of feed when operating under ideal conditions.)

If the control signal is to be derived from the power consumed by the mill motor, the circuit components necessary to produce the control signal will consist essentially of a watt-meter circuit applied across the power input lines to the mill motor and producing a voltage proportional to the motor input and a rectifier. A suitable system is illustrated in Figure 5 which shows an electronic watt-meter connected in the power lines to the mill motor from which is produced as output a rectified voltage proportional to the power input to the mill motor.

Reference signal The circuit components represented by the box 11 in Figure 2 consist essentially of a means for providing a regulated voltage which can be adjusted to a predetermined desired value and a rectifier. For instance, it may be convenient to use a voltage regulator which receives as input the standard 115 volts line voltage and produces as output a regulated voltage of say 210 volts, a rectifier, and a potentiometer which may be set at a desired value to give as final output a desired rectified voltage which may be used as a reference signal.

Comparator The circuit components represented by the comparator box 12 may be of any conventional type. For instance, the comparator may consist of a simple bridge circuit, or alternatively an electronic grid which may or may not, depending upon the circumstances, be connected so as to form an integral grid system with components of the amplifier. The only important feature of the comparator circuit is that it must produce as output a signal which is proportional to the difierence between the reference signal and the control signal and which has a sense which is opposite for opposite values of the algebraic sum of the control signal and the reference signal.

Amplifier Any suitable amplifier may be used which will fulfil the functions required in the particular application, the function of the amplifier simply being to amplify the signal produced by the comparator to a suificient extent so that it may eifect control of the particular type of feeder being used. The amplifier may, and in the preferred instance is, integrally associated with the comparator grid system.

Proportioning feeder There are various types of proportioning feeders which are available on the market, perhaps the commonest type being the electromagnet pulse type feeder of which a typical example is the type manufactured by the Syntron Company of Homer City, Pennsylvania, United States of America. This type of feeder feeds solids from the bottom of a bin along a plate which is vibrated by a magnetic pulse with an amplitude which varies as the amount of power fed to the feeder is varied. The amount of material fed is a function of the amplitude of the vibration of the feeder plate. Another suitable type of proportioning feeder consists of a variable speed conveyor belt arranged beneath the feed bin in such a way that, as the belt moves, a relatively constant load of ore per foot of belt is fed from the bin. In this type of feeder, the rate of feed to the mill is approximately proportional to the speed of the belt.

It will be appreciated that in many cases it will, in following normal procedures, be desirable to store mill feed closely adjacent to the site of the mill and to store coarse feed and fine feed separately in order to overcome segregation problems and maintain a substantial unifonnity of particle size distribution in the feed actually delivered to the mill. Thus, it is to be understood that the proportioning feeder illustrated diagrammatically in Figures 2 and 3 may be a single feeder where there is no separate storage of feed, or on the other hand, it may represent a battery of feeders feeding simultaneously from two or more separate storage bins. A typical example of the latter type of system is described in my copending application Serial No. 203,861 filed January 2, 1951, where coarse and fine feeds are fed in predetermined proportion from two separate storage bins in order to maintain a feed to the mill which has at all times at least a minimum pencentage of a selected plus size of material. In such cases, the amplified difference signal will be proportioned between the various feeders in accordance with the predetermined proportionate rate of feed of the feeders in any well known way. The present invention is not concerned with the variation in the proportion of coarse to fine which is fed to the mill, and it is, therefore, not thought necessary to describe such means in detail herein. It might be pointed out, however, that one suitable means is illustrated in my copending application Serial No. 282,505 filed April 15, 1952.

Monitoring signal As indicated previously, the monitoring signal may either be a power derived signal or a sound derived signal, depending upon the source of the control signal. The monitoring signal is applied to the system in a manner which prevents the system from creating conditions wherein a particular condition of the mill operation is exceeded. The commonest condition in connection with which it may be desired to affect monitoring is the rated power load of the mill motor. However, certain other conditions such as the capacity of the follow-up metallurgical circuit or the ultimate capacity of the feed supply circuit may also impose limitations on the operation of the mill, and render monitoring desirable, and it may, when operating under certain conditions be desired to monitor a power derived control signal on the basis of a desired charge volume as represented by a signal derived from sound :emanating from the mill.

The commonest instance where monitoring will be-required will be in connection with the rated power of the mill motor, and in this connection a convenient means of applying the monitoring to the control system is illustrated in Figure 6 which will be described in detail below.

The normal controlling signal is received at terminal 60, is compared with the reference signal received at terminal 61 across a potentiometer 62, the centre tap 62A of which is connected to the grid 63 of the first half of tube 64. The potential of this grid with respect to the cathode 65 determines the platecurrent of this half of the tube and hence the potential of point 66, since there is anIR drop across resistor 67 porportional to the plate current. Under normal operating conditions, point 66 will be 50 volts negative with respect to terminal 68 and approximately 50 volts positive with respect to terminal 69. Since 50 volts can not be put on the grid 70A of tube 70, this voltage is dropped across resistor 71 and approximately 100 volts dropped across resistor 72 between point 73 and 74.

Under such conditions, tube 70 passes suflicient current through terminal 75 to the vDJ C. winding of a saturable core reactor (not shown) connected between 75 and 68 so as to control the output of that reactor to the thyratron tubes.

The monitoring signal from watt meter 76 (see Figure is received at terminal 77 and is compared in rheostat 78 with an arbitrary reference which is received at terminal 77A, the centre point 78A of rheostat 78 picks up the difference and is connected to the grid 79 of the second half of tube 64. Since the cathode 80 of the second half of tube 64 is connected to 69 under normal conditions, the grid 79 will be negative with respect to cathode 84) and no current will flow, but so soon as the monitoring signal voltage increases or becomes positive with respect to terminal 69 and hence to the cathode 80 of the second half of tube 64, current flows from the plate 81 through the second half of the tube. Thisrincreased current'flow increases the voltage drop across resistor 67 in efiect making point 66 more negative, and hence making the grid 7 ilA of tube 70 more negative. This reduces the plate current to terminal 75 hence to the D. C. winding of the saturable core reactor so that the output of the reactor is reduced in proportion to the reduced current through tube 76.

Terminals 82 and 83 are connected across the normal 115 v. A. C. supply and provide heating current through transformer 86 to the elements 84 and 85 of the tubes 64 and 70 respectively.

Terminals 87 and 88 may be connected through capacitorsysterns to increase the stabilityof the system and reduce hunting if required.

Throughout the specification and claims, I refer to the datum or reference signal as being similar to the electrical signal which is produced and which is functional to an operation condition within the mill. By similar, I mean that the datum or reference signal and the electrical signal must be capable of being compared to produce a diiference signal; that is to say, they must be generally the same type of signal, and their values must be expressed electrically in an analogous manner. For instance, in the description, it will be observed that the signals used are all D. C. pulses obtained by rectifying a 60 cycle A. C. voltage. V i

It will be observed also that I have spoken of continuously producing both the electrical signal and the similar datum signal, and continuously comparing the two'to produce a diiference signal. By continuouslyjl mean that the whole operation of control is carried on over an extended period of timewhether the frequency of pulses (where the signals used are a pulse type of signal) is long or short. Thus, 'if instead pf having a pulse of voltage every X of a second as is the case where-the signals are rectified cycle D. C. voltages a pulse type signal wereto be used which had a frequency measured in seconds, the control system described would obviously function satisfactorily. The word continuously is, therefore, to be understood as characterizing the production and utilization of the signals concerned whether these signals themselves be of intermittent nature or not. 7

In the specification and claims, I refer to the total charge. By this phrase, I mean the total of all the material in the mill including such reduction media as may be present in addition to the material which is undergoing comminution.

' What I claim as my invention is:

1. A method of controlling the operation of dry material reduction mills of the rotating drum type whereby to maintain said operation substantially constant at a desired operating point corresponding to a particular point on a. pre-e-stablished grinding curve, which method comprises; continuously producing an electrical signal which varies with an operating condition in the milling system which, at the desired operating point is subject to appre' ciable change for small changes in total charge volume in said mill; continuously producing a similar datum signal equal in value to an established value of said electrical signal at said desired operating point; continuously comparing said electrical signal with said datum signal to produce a difference signal and varying in a continuous manner-the rate at which feed material is supplied to said mill inaccordance with the sense and magnitude of said difference signal.

2. A method of controlling the operation of dry material reduction mills of the rotating drum type whereby to maintain said operation substantially constant with a relatively large total charge volume in said mill, which method comprises; continuously producing an electrical signal which varies with sound emitted by said mill; continuously producing a similar datum signal equal in value to an established value of said electrical signal at the operational point of said mill which it is desired to maintain;

continuously comparing said electrical signal with said datum signal to produce a difference signal and varying in a continuous manner the rate at which feed material isrsupplied to said mill in accordance with the sense and magnitude of said difference signal.

3. A method as defined in claim 2 wherein the sound with which said electrical signal varies is predominantly of a frequency higher than 2,000 cycles.

continuously comparing said electrical signal with said.

datum signal to produce a difference signal and varying in a continuous manner the rate at which feed material is supplied to said mill in accordance with the sense and magnitude of said difference signal. a

5. A method of controlling the operation of dry material reduction mills of the rotating drum type whereby to maintain said operation substantially constant with a relatively low total charge volume in said mill, which method comprises; continuously producing an electrical signal which varies with the power drawn by the mill motor, continuously producing a similar datum signal equal in value to an established value of said electrical signal at the operating point of said mill which it is desired to maintain; continuously comparing said electrical signal with'said datum signal to produce a difference signal and varying in a continuous manner the rate at which feed material is supplied to said mill in accordance with the sense and magnitude of said difference signal.

6. A method of controlling the operation of dry material reduction mills of the rotating drum type whereby to maintain said operation substantially constant at a desired operating point corresponding to a particular point on a pre-established grinding curve, which method comprises; continuously producing an electrical signal which varies with an operating condition in the milling system which, at the desired operating point is subject to appreciable change for small changes in total charge volume in said mill; continuously producing a similar datum signal equal in value to an established value of said electrical signal at said desired operating point; continuously comparing said electrical signal with said datum signal to produce a difference signal; varying in a continuous manner the rate at which feed material is supplied to said mill in accordance with the sense and magnitude of said difierence signal, and applying a similar monitoring signal to reduce the rate at which feed material is supplied to said mill, regardless of the sense and magnitude of said difierence signal whenever a predetermined operating condition of said milling system is exceeded.

7. A method as defined in claim 6, wherein the monitoring signal varies with the rate of production of the mill and wherein the predetermined operating condition corresponds to a desired rate of production of the mill.

8. A method of controlling the operation of dry material reduction mills of the rotating drum type whereby to maintain said operation substantially constant with a relatively large total charge volume in said mill, which method comprises; continuously producing an electrical signal which varies with an operating condition of said mill selected from the group consisting of sound and vibration; continuously producing a similar datum signal equal in value to an established value of said electrical signal at the operating point of said mill which it is desired to maintain; continuously comparing said electrical signal with said datum signal to produce a difference signal; varying in a continuous manner the rate at which feed material is supplied to said mill in accordance with the sense and magnitude of said difierence signal; and applying a similar monitoring signal functional to power input to the mill motor to reduce the rate at which feed material is supplied to said mill regardless of the sense and magnitude of said difierence signal whenever the power input to the mill motor exceeds a predetermined value.

9. A method of controlling the operation of dry material reduction mills of the rotating drum type whereby to maintain said operation substantially constant with a relatively small total charge volume in said mill, which method comprises; continuously producing an electrical signal which varies with power input to the mill motor; continuously producing a similar datum signal equal in value to an established value of said electrical signal at the operating point of said mill which it is desired to maintain; continuously comparing said electrical signal with said datum signal to produce a difierence signal; varying in a continuous manner the rate at which feed material is supplied to said mill in accordance with the sense and magnitude of said difierence signal; and applying a monitoring signal functional to sound emitted by said mill to reduce the rate at which feed material is supplied to said mill, regardless of the sense and magnitude of said difference signal whenever the sound emitted by said mill falls below a predetermined value.

References Cited in the file of this patent UNITED STATES PATENTS 2,001,543 Payne May 14, 1935 2,136,907 Roder Nov. 15, 1938 2,235,928 Hardinge Mar. 25, 1941 2,381,351 Hardinge Aug. 7, 1945 2,491,466 Adams Dec. 20, 1949 

